Catheters of various types are used to drain fluid from different areas of the body of a patient. One application of such catheters is for the treatment of hydrocephalus, a condition where cerebrospinal fluid (CSF) collects in the ventricles of the brain of a patient. CSF is produced by the ventricular system and is normally absorbed by the venous system. However, If the CSF is not absorbed, the volume of CSF increases thereby elevating the intracranial pressure. This excess CSF can result in abnormally high epidural and intradural pressures. Left untreated, hydrocephalus can result in serious medical conditions, including subdural hematoma, compression of the brain tissue and impaired blood flow.
Various drainage catheters or shunt systems have been developed to remove the excess CSF and to discharge the fluid to another part of the body, such as the peritoneal region. By draining the excess fluid, the elevated intracranial pressure is relieved.
Generally, fluid shunt systems include a valve mechanism for controlling or regulating the flow rate of fluid through the system. An illustrative shunt system includes a valve mechanism in fluid communication with a brain ventricular catheter. The ventricular catheter is inserted into a ventricle of the brain and a peritoneal catheter is inserted into the peritoneal region for discharge of the fluid. Exemplary shunt systems include U.S. Pat. Nos. 4,332,255, 4,387,715, 4,551,128, and 3,886,948, all of which are incorporated by reference herein.
FIG. 1 shows a prior art shunt system 10 having a ventricular catheter 12 inserted through a hole 14 in the skull 15 of a patient. The catheter 12 is advanced through brain tissue 16, and into a ventricle 18 of the brain where excess CSF is present. The catheter 12 is coupled to an inlet end of a shunt valve 20 and a drainage catheter 22 is coupled to an outlet end of the shunt valve. The shunt valve 20 is typically implanted under the scalp (not shown) of the patient The shunt system is operative to drain excess CSF from the ventricle to another part of the body, such as the right atrium, peritoneal cavity, or other locations in the body.
Shunt systems typically permit fluid flow only when the fluid pressure reaches a threshold pressure for the shunt valve. The fluid flow rate is proportional to the pressure at the valve mechanism. Thus, for a pressure slightly greater than the threshold pressure, the flow rate is relatively low. As the pressure increases the flow rate through the shunt system concomitantly increases. At pressures significantly greater than the threshold pressure, a maximum flow rate for the system is reached. Fluid flow normally continues until the intracranial pressure has been reduced to a level less than the threshold pressure, subject to any hysteresis of the device.
The threshold pressure that allows fluid flow through a shunt system must often be adjusted. For example, a surgeon may initially select a relatively low threshold pressure to trigger fluid flow. Over time, the initial threshold pressure may not be ideal. For example, it could lead to excess fluid flow, creating an undesirable overdrainage condition in which too much fluid is drained from the ventricle. Such a situation may give rise to a need to increase the threshold pressure to afford a fluid flow rate that is balanced to avoid both excessive intracranial pressure and overdrainage conditions.
The hydrocephalus shunt system is surgically implanted, and adjustment of the threshold pressure often requires the system to be surgically removed, adjusted, and then surgically implanted again. If the valve is not adjusted correctly, or if the intracranial pressure of the patient changes over time, then the valve must be re-adjusted, i.e., surgically removed and re-implanted.
One prior art shunt valve that allows threshold pressure adjustment without removal of the device is disclosed in U.S. Pat. Nos. 4,615,691 and 4,772,257, both of which are incorporated by reference herein. These patents disclose a cerebrospinal fluid shunt valve that is externally adjustable by means of a programming device. The shunt valve includes a stepping motor having rotor and stator elements. The stator elements are composed of a magnetically soft and permeable material shaped and positioned with respect to the rotor. The external programming device applies a magnetic field causing the rotor to rotate about a central axis so as to adjust the threshold pressure.
The programming device includes a surface having a groove therein for accommodating a protrusion on the scalp of a patient caused by the implanted shunt valve. The device includes a plurality of generally coplanar electromagnets disposed about a central axis. To adjust the shunt valve, the programming device is first placed against the head of the patient so that the scalp protrusion is within the groove of the device. An operator verifies the longitudinal position of the shunt valve within the groove to enable proper pressure adjustment. The central axis of the electromagnets and rotor should be substantially coaxial to ensure accurate programming of the valve. Once the proper position is achieved, the electromagnets in the programming device are energized to apply a magnetic field to actuate the stepper motor and select a predetermined threshold pressure for the valve mechanism.
Proper alignment of the axes can be difficult to achieve because an operator cannot see the rotor or the stator elements. This difficulty is further compounded by the rather small size of these elements relative to the electromagnets. Although the device can tolerate some degree of axial misalignment, the reliability of the external programming device degrades as the degree of misalignment increases.
Misalignment can occur due to rotation of the shunt valve under the patient's scalp. The external programmer must be placed adjacent to the top surface of the shunt system. Any rotation or rolling of the system will misalign the system, making external adjustment difficult or impossible.
The shunt valve is typically disposed in a generally rectangular housing with rounded corners. The valve has an aspect ratio with respect to height and width that is approximately unity, making it somewhat prone to rolling. The valve can also be housed within a generally circular silicone housing that further contributes to a rounding of the valve, creating further rotational instability. Thus, the shunt valve and stepper motor rotor can easily displace from a programmable position where the rotor axis is normal to a portion of the skull where the valve is located. Under certain conditions, such as a well placed impact, the valve can rotate forty-five degrees or more, thus rendering the programming device useless and requiring surgical removal of the valve in order to effect pressure adjustment.